1. Field of the Invention
This invention relates to pneumatic tires for heavy duty road vehicles and more particularly to a highly durable pneumatic tire comprising a bias laid carcass construction formed of superimposed stacked plies each including rubberized organic textile cords.
The tire according to the invention is mainly used for large type trucks, construction vehicles, agricultural vehicles, industrial vehicles, airplanes or the like.
2. Description of the Prior Art
The above mentioned kind of tire, that is, the tire comprising the bias laid carcass construction formed of superimposed stacked plies each including the rubberized organic textile cords has a number of advantages, but has a few drawbacks. The most important drawback consists in that the number of carcass plies must be considerably increased in order to give the tire that strength which is required when the tire is in use.
In a tire comprising a carcass body formed of the most popular nylon cord of 1,260 denier/2 strands, the number of carcass plies becomes usually 10 to 20, and in a large tire the number of these carcass plies eventually exceeds 40.
It is clear that the use of such large number of the superimposed stacked carcass plies results in a decrease in production efficiency and increase in manufacturing cost of the tire if compared with a steel radial tire which makes use of only one carcass ply even when the tire is large in size.
Heretofore it has been proposed to make the diameter of the cord of the carcass ply large and hence make the number of filaments large so as to increase the strength per cord in order to decrease the number of the carcass plies while maintaining the strength required for the tire. For example, if a carcass cord of 1,890 denier/2 strands is used instead of a carcass cord of 1,260 denier/2 strands, the former becomes 3/2 times higher in the cord strength and becomes about 3/4 times smaller in the number of plies than the latter. The former does not become 2/3 times smaller in the number of plies than the latter as expected by calculation owing to the fact that the number of cords must be reduced for the purpose of maintaining a certain gap between the cords. As a result, it is possible to reduce the required number of plies from 30 to the order of 22.
Such conventional method has the drawback that the carcass plies reduced in number tend to increase a cord breaking failure which is a complete tire failure.
The cord breaking up failure means a tire failure in which the cords of the carcass ply become broken due to the following reasons. That is, the cord breaking failure seldom occurs when the tire is in use under proper conditions, but occurs when the internal pressure of the tire becomes extremely small or the tire is subjected to heavy load or sudden shocks or the tire size is erroneously selected and hence the carcass strength becomes insufficient.
Experimental tests have yielded the result that, if the carcass is the same in strength, the use of a cord having a large diameter induces the cord breaking failure more frequently than the use of a cord having a small diameter.